Was it just because they thought every one was eventually going to fight for independence anyway?
For the most part, they didn't, until giving up the Empire had become inevitable. They went to great lengths to try and sustain the Empire till they simply didn't have the resources to keep it running.
One example of this is India. In India's case, you have a long history of bad faith by the British. The British expansionism itself was a long history of broken promises and aggressive policies. Still all that aside, by the 20th century, India was a great provider of natural resources. India was moreover an export country, with its raw materials being sold all over, providing a trade surplus, which Britain used to balance its trade deficit as a predominantly Import centered economy. In effect, India exported raw goods, which were either sent to Britain at knocked down rates for processing (ie the Manchester cloth angle) or were sold competitively. The money made, was used to excuse Britain's imports, even as India was forced to absorb Britain's finished goods. Oh and also, almost every Native Prince was bound by subsidiary alliances, which as the names suggests, was used to subsidize British forces.
Now, this becomes important in the context of independence. India had a long running Nationalist Freedom movement. By the time World War I rolled around, it was pretty much pan national. The British made a series of promises to the various Indian parties regarding greater autonomy. In particular, they promised fiscal autonomy in the aftermath of the war, as long as cooperation was rendered.
After the war? Zip. They walked back on all their promises of autonomy, retained most policies of racial discrimination. Some token gestures were made, such as fiscal autonomy in theory, but this didn't provide any actual relief. Instead Britain set about coercing India into its system of "Imperial Preference" wherein India was mandated to provide "free trade" to other states within the British Empire, but was forced to throw up tariffs to those outside, notably the Japanese. This was in contravention to the desires of India's own nascent industries, which is why when India "voluntarily" attended and agreed to the conventions of the Ottawa Conference, the Viceroy ensured that no actual Indian Industrial representatives were allowed to attend. Indeed, after large scale Indian support during the war, Indians were rewarded by massacres such as at Jallianwala Bagh and a clampdown on freedoms.
Britain gave up India after World War II, because it was abundantly clear that they simply could not hold on. It is worth remembering, that even as Britain maintained its financial obligations to the United States after the war, it essentially used "freedom" to wipe out all debts it owed to India it had raised during the War. A somewhat large sum was paid out, but it was nowhere near the total costs. India for example had "voluntarily" given flat out gifts to Britain to sustain the war, not to mention the vast sums of costs incurred in men and material costs that India bore. This is all independent of more moral issues of reparations for events such as the Bengal Famine. To consider the contrast - the final Lend Lease payments only ended sometime in 2006. Yet British obligations to India effectively ended with Independence.
I also think that not very long after Independence, Britain began slowly becoming more and more hostile towards immigration. Whereas under the Empire people were free to move to Britain as long as they could muster the resources, increasingly Britain began seeking to restrict movement. The more recent policies we see, such as May's "Hostile Environment" is only a recent peak of what I think is a long running trend of wanting to wash their hands in all ways from Empire since they didn't have the financial wherewithal to hold it. At the end of the day it is important to remember that the bulwarks of Britain's Imperial resources had been the Opium Trade and its Industrial economy having captive markets. As Indian industry took off, and resistance grew to British goods, these trade goods started to wither, Britain slowly lost the very basis of its Empire.
Some sources you might wish to pursue if you want a more detailed examination of this subject.
Basudev Chatterji - Trade, tariffs, and empire: Lancashire and British policy in India, 1919-1939
Prasannan Parthasarathi - The Transition to a Colonial Economy
Gallagher and Robinson - The Imperialism of Free Trade
Cain and Hopkins - British Imperialism
Tirthankar Roy - Traditional Industry in the Economy of Colonial India
You should ofcourse try and read up more works by these authors if you can, but these lie at the foundation of the different points I've made here in my post, so if you're interested in seeing where I'm coming from, you can read these.
The reason I've discussed India is because I'm an Indian Historian. But the point here is, from what I understand the same is true of much of Britain's holdings. They had to give them up in the face of resistance, and their inability to hold on. I've done a brief survey of South African history, which suggests a similar issue, especially given the controversies on autonomy which led to the Boer War and then following that independence by the mid 20th century. Africa was slower in gaining it, but slowly it too broke away as Britain shrunk. As far as I can tell, Canada, Australia and New Zealand were the only really voluntary give ups by the Empire, and racialism might have played a role there.
There was always a difference in policy between the 'Old Dominions' (Canada, Newfoundland, Australia, New Zealand, South Africa, and eventually the Irish Free State) and the colonies of tropical Africa and the Pacific. /u/boringhistoryfan has given a great comment about India's situation (belying his name!), a country which sat uncomfortably between these two groups, never fully included in either, and a source of endless debate from contemporary imperialists at the time as to where it fit in. I can only speak to the settler colonies.
The Old Dominions were united to Britain by more than simple force of arms. They had majority-White populations that had their origins in British settlement (with the exception of South Africa); English was spoken by a substantial majority of the population, not just the political leadership; they held to the same political and religious institutions as a Londoner (with the exception of the Free State); they were largely loyal to the British monarch. While South Africa and the Free State had been in open conflict with the Crown, those conflicts had been resolved by the early 1920s and they had been welcomed into the imperial system despite their differences with the rest of the Commonwealth. While I’m not fully on board with the ‘Greater Britain’ and ‘British World’ theories, Britain and its Dominions shared enough informal bonds to allow contemporaries see a difference between Australia and British East Africa.
The intention, at least publically, had always been to grant the colonies ‘responsible government’ as and when the British decided they were ready. For the settler colonies, self-government and autonomy came relatively rapidly due to the perceived trustworthiness and loyalty of their populations, due to these bonds; Canada achieved this first in the mid-19th century, followed by New Zealand and the six colonies that would eventually make up the Commonwealth of Australia, and then Cape Colony. After a few confederations and unions, the title of Dominion (which had been Canada’s official title since 1867) was gradually granted to these colonies. With the establishment of the Irish Free State (one of Britain’s conditions for peace was the state receiving Dominion status) the British Commonwealth took official form. From 1887 the heads of government from these colonies gathered every few years to discuss their grievances and consider new policies at the Colonial and Imperial Conferences. After significant contributions from the settler colonies in both the Boer War and the First World War, the Dominions demanded further concessions from the Crown, much like /u/boringhistoryfan stated India had. Dominion involvement in the war, including a wartime Imperial Conference, gave the Dominion Premiers a significant political boost.
When the newly-formed Republic of Turkey began to agitate against the Greek and Allied occupation of its territory in 1922, the British Government threatened to declare war on behalf of her Empire, as it always had, only to be publically rebuked by Canadian Premier Mackenzie King. Ottawa rejected London’s right to pull Canada into a war not of their choosing, championing the authority of the Canadian Parliament. At the signing of the Lausanne treaty, resolving the crisis, the Dominions refused to take part, and declared that Britain was signing only for herself, as they had not been consulted. In 1926, the Balfour Declaration (not that one) made it clear that the Dominions were equal to one another and Britain; no one had the authority to make any other bend to their will, and all were united in their common allegiance to the Crown. This statement was put into effect with the Statute of Westminster in 1931, which formally removed the right of the Westminster parliament to legislate on behalf of their Dominion counterparts. From here on, the Commonwealth was a purely voluntary institution (although the possibility of secession was hotly debated). Those Dominions so inclined gradually stripped the vestiges of imperial rule away, while others let them be.
For the white dominions, independence (in every way that mattered) was mostly a gradual, negotiated process. Their colonial subjects looked, talked, and worshipped much the same as British politicians, and so compromise rather than military force was often the preferred method of resolving constitutional differences.
Sources: